Many refineries recover valuable products from the heavy residual oil that remains after refining operations are completed. This recovery process, known as delayed coking, produces valuable distillates and coke in one or more large vessels known as coke drums or coking vessels. As used herein, coking drums and vessels are used interchangeably. The operating conditions of delayed coking can be quite severe. Normal operating pressure typically range from about 15 to about 60 pounds per square inch, and the feed input temperature may be over 900.degree. F.
Coke drums are typically large, cylindrical vessels having a top head and a frustoconical bottom portion fitted with a bottom head. Coke drums are usually present in pairs so that they can be operated alternately. Thus, while one coke drum is being filled with residual oil and heated, the other drum is being cooled and purged of up to several hundred tons of coke formed during the previous recovery cycle. Purging a drum of coke is often referred to as "coke recovery".
Coke recovery begins with a cooling step in which steam and water are introduced into the coke filled vessel to complete the recovery of volatiles and to cool the mass of coke. The vessel is then vented to atmospheric pressure and the top head (typically a 4-foot diameter flange) is unbolted and removed. The vessel is drained and the bottom head (typically a 7-foot diameter flange) is unbolted and removed. A hydraulic coke cutting apparatus is inserted into the vessel to cut the coke. The hydraulically cut coke falls out of the vessel and into a recovery chute to be channeled to a coke pit or pad. Typically, the recovery chute replaces the removed bottom head so that coke falling through the opening created by removal of the bottom head immediately enters the recovery chute. While ideally all of the coke is captured by the chute and falls through to a coke pit below, in practice there is spillage, and essentially all of the coke is taken to be channeled through to the coke pit as long as no more than a few percent of coke spills onto the floor. However, it is common to have at least some spillage whenever the head is removed, and such spillage is undesirable both because it results in extra work in cleaning up the spillage, and because it poses a threat to workers. Spillage frequently occurs during bottom head removal, and occasionally occurs after head removal but before the coke chute is positioned, especially when shot coke is produced.
Attempts to prevent spillage and to reduce risk to workers have been made, and can generally be categorized as (1) automating the opening or deheading of the coking vessel, and (2) automating the positioning of the coke chute. Automating the deheading of the coking vessel decreases the risk to workers because they need not be present during the initial opening of the vessel when spillage frequently occurs. However, decreasing the risk to workers in this matter does not eliminate the spillage during the opening process. Moreover, if the positioning of the coke chute is not automated, the workers are at risk while positioning the coke chute. Even with an automated chute positioning system, there is a chance that spillage, itself undesirable, will occur prior to the chute being positioned.
Thus, there is a further need for an improved method and system which will allow coke to be discharged from a coking vessel without placing workers at risk such as by requiring workers to position a coke chute after opening an outlet in the coking vessel, and which will prevent spillage while opening an outlet in the coking vessel. Other and further objects and advantages will appear hereinafter.